Not sure if this is any help, but a while ago, I was able to get multiple simultaneous taps on a touch tablet in Windows. I think it was fullscreen, so the coordinates were probably not Window-relative. It was also a quick test, so not aware of many limitations:

You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot vote in polls in this forumYou cannot attach files in this forumYou can download files in this forum